1. Field of the Invention:
This invention relates to a safety device for a jacking system including a plurality of jacks which are designed for simultaneous operation. More particularly, it is a device which ensures the simultaneous operation of the jacks when a cargo table or tables are lowered.
2. Description of the Prior Art:
There is known a jacking system including a plurality of jacks which are designed for simultaneous operation. It is a system which is used, for example, for a cargo handling job, or for lifting a motor vehicle when repairing it. A typical system of this category comprises a pair of jacks which are connected to each other by a mechanism for raising and lowering the two jacks simultaneously. It also includes a horizontal table or tables supported on the jacks for carrying a cargo thereon. Each jack is provided with a pawl and rack mechanism for holding the table in a raised position. A typical pawl and rack mechanism comprises a pair of racks and a pair of pawls each of which is normally held in engagement with one of the racks for holding the table in its raised position, but is disengageable therefrom for allowing the table to be lowered.
A serious problem can, however, occur if the pawls in either of the jacks are not properly disengaged from the racks when an attempt is made to lower the table or tables. One of the jacks stays in its raised position, while the other jack is lowered. As a result, the table is inclined, or the tables are positioned at different levels of height. This results in the inclination, displacement or dropping of the cargo on or off the table or tables. There is every likelihood that the cargo may cause a safety hazard to the jack operator.
Various types of safety devices are, therefore, known. They include a device which detects the position of the table or tables and transmits a signal for actuating a mechanism for lowering the table or tables only when the table is not inclined, or when the tables stay at the same level of height. This device is, however, highly complicated or sophisticated. It is expensive and adds considerably to the cost of the jacking system as a whole.
Another known device includes a limit switch which functions to operate a mechanism for lowering the table or tables only when the pawls are all disengaged from the racks. This device, however, lacks reliability in operation, as the terminals of the limit switch often fail to make proper contact, or as a limit switch actuator often fails to work properly.